1. Field of the Invention
The present invention is in the field of monolithic chip capacitors, especially chip capacitors adapted to be connected directly to circuit boards as opposed to capacitors having flexible leads extending therefrom.
2. The Prior Art
Current electronic devices are making progressively increasing use of chip capacitors because of their compactness and inherent reliability.
In accordance with a conventional means for employing such capacitors, the same are provided with two or more termination portions at their end edges and are mounted directly to a substrate of alumina or epoxy-filled fiberglass carrying conductive lands on the surface thereof. Reflow solder connections are effected between the capacitors and the lands, the terminations being in direct contact with the lands. The solder provides both electrical and mechanical connections of the article to the substrate.
In the course of soldering, and in many instances in use, the device incorporating the circuit board and capacitors are subjected to thermal excursions.
Due to the differential coefficients of expansion of the various connected materials and due further to the relative fragility of the capacitors, particularly at the interface between the termination and the capacitor electrodes, a relatively high incidence of capacitor failure has been experienced despite the inherent reliability of the capacitor device. In order to obviate such failure, attempts have been made to introduce a compliant connection at the interface between the capacitor termination and circuit board, such that the differential shrinkage and expansion of the components will not exert undue stresses on the fragile elements of the device but, rather, will be absorbed by flexure of the compliant connector.
While the utilization of such compliant connectors provides a capacitor having an extremely high degree of reliability, the application of the compliance member, such as a tab, or the like has added a significant element of cost increase by virtue of the presence of an additional part, together with the operations necessary to the applying of the part.
An additional loss in reliability of the capacitor is occasioned by the presence of voids in the area between the capacitor electrodes and opposing termination. Such voids have been determined to be present in the capacitor body, in part as a result of the fact that the electrodes between the dielectric layers are of finite thickness. Thus, when a unit of compressive force is applied, utilizing conventional techniques, across the opposed surfaces of the capacitor, greater pressures are developed in the areas of greatest thickness, e.g., the central area in registry with overlapped electrodes of opposite polarity, than in the marginal areas wherein no overlapping of the electrodes occurs. As a result of insufficient compression of the marginal areas, weak spots or voids are present extending from the end of the electrodes terminating short of the end of the capacitor to the termination material at the end of the capacitor, resulting in a capacitor susceptible to voltage breakdown and/or degradation of insulation resistance, with consequent changes in value.